Not Applicable
The present invention is directed generally to network, transmission and communication systems. More particularly, the invention relates to optical information network, transmission and communication systems and optical components, such as cross connect switches, add/drop devices, demultiplexers, and multiplexers, for use therein.
The development of digital technology has provided electronic access to a vast amount of information. The increased access to information has fueled an increasing desire to quickly obtain and process the information. This desire has, in turn, placed ever increasing demands for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems linking the processing equipment (i.e., telephone lines, cable television (CATV) systems, local, wide and metropolitan area networks (LAN, WAN, and MAN)).
In response to these demands, many transmission systems in use today either have been or will be converted from electrical to optical networks. Optical transmission systems provide substantially larger information transmission bandwidths than electrical systems, which provides for increased information transmission capacities.
Early optical transmission systems were developed as space division multiplex (SDM) systems. In early SDM systems, one signal was transmitted as a single optical wavelength in each waveguide, i.e., fiber optic strand. A number of waveguides were clustered to form a fiber optic cable that provided for the transmission of a plurality of signals in spaced relationship.
As transmission capacity demands increased, optical transmission and receiving equipment was developed that provided for time division multiplexed (TDM) transmission of a plurality of distinct optical signals in a single waveguide. Optical TDM systems are generally analogous to electrical TDM systems in that the signals are transmitted on a common line, but spaced in time. The transmission of the signals is in a known sequence allows the plurality of distinct signals to be separated after transmission.
A problem with TDM transmission is the transmission bandwidth in the waveguide increases with each additional multiplexed signal. For example, information can be transmitted through a waveguide via a first series of optical signals separated in time by an interval xcex94t. Additional information can also be transmitted over the same waveguide using a second series of optical signals during the time interval xcex94t by merely offsetting the transmission of the first and second series of signals in time. While an optical signal in each series is only transmitted through the waveguide every xcex94t interval, two signals, or n signals in the general case, are passing through the waveguide during each interval. Therefore, the overall transmission rate in TDM systems increases directly with the number of signals transmitted.
Signal transmission rates in fiber optic waveguides are generally limited by the interactions between the optical signal (i.e., light pulse) and microstructural features of the waveguide material. As the transmission rate is increased, signal dispersion in the fiber and other transmission effects deleterious to signal quality begin to occur as a result of the interactions.
Optical signals are typically transmitted in wavelengths that minimize dispersion in the fiber. For example, older optical systems are commonly operated around 1310 nm and employ SMF-28 fiber manufactured by Corning, or its equivalent, which has minimum dispersion at or near 1310 nm. Another type of fiber, known as dispersion shifted fiber, has its minimum dispersion at or near 1550 nm. A third type of fiber sold by Corning as LS fiber and by Lucent Technology as TrueWave has its minimum dispersion at or near 1550 nm. In addition to having different minimum dispersion wavelengths, each fiber has varying immunity to other signal degradation mechanisms, such as four wave mixing, at increased transmission rates.
The transmission rates at which the signal quality begins to degrade are substantially lower ( less than 40 Gbps) than the capacity of the transmission and receiving equipment. Therefore, TDM systems, which increase capacity by increasing transmission rates, generally have only a limited potential for further increasing the capacity of optical transmission systems.
The development of wavelength division multiplex (WDM) transmission systems has provided a way to increase the capacity of optical systems without encountering the waveguide limitations present in TDM systems. In a WDM system, a plurality of optical signals including information carrying wavelengths are combined to produce a multiple wavelength signal that is transmitted through the system to a receiver. After the multiple wavelength signal is received, the information carrying wavelengths are separated from the multiple wavelength signal and provided to a corresponding plurality of destinations. Unlike TDM systems, only one WDM signal is transmitted during a time interval xcex94t, although each WDM signal contains a plurality of signals including information carrying wavelengths.
Also unlike TDM systems, the waveguide material does not realistically limit the information bandwidth that can be placed on a single optical fiber in a WDM system. One skilled in the art can also appreciate that the number of wavelengths that can be used to transmit information over a single waveguide is currently limited by the complexity of the transmission and receiving equipment required to generate, transmit, receive, and separate the multiple wavelength signal.
Currently, many optical transmission systems must convert the optical signal to an electrical signal during transmission to perform transmission functions, such as signal amplification and switching. The optical to electrical conversion, and vice versa, substantially limits the overall transmission speed of the network, and increases transmission losses in the network. Thus, it has been an industry goal to develop optical amplifiers and optical cross-connect switches to provide for high speed, all optical transmission systems.
The development of optical fiber amplifiers produced by doping the optical fiber with Erbium ions (Er3+) or other elements has allowed for the elimination of electrical amplifiers and the requisite time delay and costs associated with signal conversion. In addition to simplifying and decreasing the cost of the equipment required to amplify a signal, optical fiber amplifiers have proven effective for amplifying a plurality of wavelengths without a commensurate increase in the complexity of the amplifier as additional wavelengths are included in the WDM signal.
Unlike optical amplifiers, optical cross-connect switches greatly increase in complexity as the number of waveguides entering and exiting the switch and the number of wavelengths per waveguide increases. As a result, the expansion of all optical systems has been somewhat inhibited by the lack of simple, efficient, and economically attractive optical cross-connect switching systems.
A number of optical cross-connect switches are based on one or more 1xc3x972 signal splitters or 2xc3x972 signal couplers used in conjunction with one or more wavelength filters, such as described in U.S. Pat. No. 5,446,809 issued to Fritz et al. The complexity of these types of switch increases not only with the number of inputs and outputs in the switch, but also with the number of wavelengths being switched. For example, if a 2xc3x972 switch is provided to switch two eight wavelength WDM input signals to two output signals, the switch would have to include 32 gratings to allow all wavelengths to be switched. However, if a 4xc3x974 switch is provided to switch four sixteen wavelength. WDM input signals to four output signals, 256 gratings will be required. In addition, the flexibility of the switch is limited because additional gratings or filters must be added to each waveguide connecting each input to each output of the switch for every wavelength that is to be switched.
Another complication is that different signals entering a switch at different input ports will often times be carried by the same wavelengths. The use of common wavelengths frequently occurs because optical signals are generally transmitted using a relatively narrow range of wavelengths that have been established by optical standards committees with the goal of minimizing transmission losses in a waveguide and allowing equipment standardization in the industry.
If two signals on a common wavelength from different inputs are switched to the same waveguide, both signals will be destroyed. The switch, therefore, must be designed to prevent the inadvertent destruction of signals transmitted to the switch on a common wavelength.
Switches can be provided that xe2x80x9cblockxe2x80x9d the switching of certain wavelengths to prevent destruction of two signals on a common wavelength. Switches can also be provided with wavelength converters that are used to change the wavelength of a signal, in lieu of blocking the signal, to prevent the destruction of the signal. U.S. Pat. No. 5,627,925 issued to Alferness et al. discloses an example of a switch thaw includes wavelength converters to provide a nonblocking switch. As expected, the use of wavelength converters adds a further degree of complexity to the design and function of optical cross-connect switches.
An alternative to adding wavelength converters to provide a nonblocking switch is to limit the wavelengths used in the system. For example, U.S. Pat. No. 4,821,255 issued to Kobrinski discloses an optical system employing transmission systems that transmit data at a different wavelength to each destination receiving system, i.e., N wavelengths for N receiving systems. In this manner, the optical system does not require a nonblocking switch and the assignment of a specific wavelength to each receiving system allows for a passive optical connection (xe2x80x9chard wirexe2x80x9d) between a transmission demultiplexer and a receiving multiplexer.
In addition, the same N wavelengths can be transmitted by each transmitting system if the receiving system is coordinated to receive a different wavelength from each transmitting system. Wavelength coordination eliminates the need for wavelength converters and allows the same transmitters and receivers to be used in the system.
A difficulty with passive switching systems is that the streamlined nature renders the switch somewhat inflexible. For example, a specified wavelength is used to transmit signals between a transmission system and a receiving system. Therefore, it may be difficult to transmit multiple signals from one transmitting system to one receiving system at any one time. It is presumably possible to assign additional wavelengths to each of N transmitter/receiver combinations; however, for each wavelength added to each system, either N2 hard wire connections must be made.
The problem of signal blockage can also be addressed by designing a system having excess transmission capacity. This would provide more available wavelengths than is required to meet current transmission requirements. However, in view of the continued expansion of communication networks the excess capacity may only be short term; therefore the ability to upgrade a system remains a desired feature of a switch.
Similarly, other optical components, such as add/drop devices, demultiplexers and multiplexers, used in optical processing nodes between the transmitter and receivers increase in complexity and cost as additional channels are added to the system. In addition, these components most likely have to be replaced when a system is reconfigured or additional channels are to be added to the system.
The continued advancement and development of communication systems is limited, at least in part, by the constraints placed upon optical systems by the current technology involved in optical processing systems. The elimination or reduction of these constraints is a primary concern of industry as the pace of communications continues to accelerate.
Accordingly, there is a need for optical systems and optical components that allow for increased network capacity and flexibility. One aspect of which is to reduce the complexity of the equipment and increase the efficiency of the transmission system.
The apparatuses and methods of the present invention address the above needs and concerns for improved optical switches and systems. An optical transmission system of the present invention includes one or more optical signal transmitters and optical signal receivers optically communicating via one or more intermediate optical processing nodes. Each optical transmitter includes one or more optical sources, such as modulated lasers, and is configured to transmit information via one or more information carrying wavelengths. Each optical receiver is configured to receive one or more of the information carrying wavelengths using one or more various detection techniques, such as direct detection using optical wavelength filters and photodiodes, or indirect detection using coherent detectors.
The intermediate optical processing nodes include optical switches, add and/or drop devices including at least one waveband selector configured to pass and substantially prevent the passage of optical wavebands that include a plurality of information carrying wavelengths from the transmitter to the receiver. The optical processing nodes provide for information management and processing in wavebands, instead of separating individual information carrying wavelengths from the signal and individually processing each wavelength. In this manner, high capacity processing of the information can be achieved without the prior complexities involved with increasing capacity. The processing of pluralities of individual wavelengths further provides for accommodating varying numbers and distributions of individual information carrying wavelengths in the system without having to reconfigure or replace system components.
In an embodiment of the present invention, the optical processing node includes a switch providing cross connections between a plurality of transmitters and receivers. Optical signals including one or more information carrying wavelengths are transmitted to optical switch input ports and are distributed to optical switch output ports by splitting and/or waveband demultiplexing the optical signals depending upon the type of waveband selector used in the switch.
Waveband selectors include at least one switch, gate, or filter, such as an erbium or mechanical switch, a Bragg grating, or a Mach-Zehnder or Fabry-Perot filter. The waveband selectors are generally configured to pass one or more optical wavebands from the input port to the output port in one mode and/or to substantially prevent the passage the optical wavebands in another mode. A signal is generally considered to be substantially prevented from passage, if the signal is sufficiently attenuated such that a remnant of the attenuated signal passing through the waveband selector does not destroy signals that have been selectively passed through the optical processing node. For example, a 40 dB attenuation of a signal will generally be sufficient to prevent cross-talk interference between remnant signals and signals passing through the optical processing node.
In an embodiment, each input signal is waveband demultiplexed to separate the input signal into waveband signals. Each waveband signal is then split and each split waveband signal passed through a switch to a respective output port. In an embodiment, an erbium doped fiber is used as the switch in the waveband selector to pass, as well as to controllably amplify or attenuate, the split waveband signal to the output port when supplied with optical pump power. In the absence of pump power, the erbium fiber absorbs the waveband signal, which substantially prevents the passage of the signal. One or more optical combiners are provided at the output ports to combine split waveband signal from the waveband selector passing optical wavebands from the input ports.
The optical signal at each input port can also be demultiplexed according to a known destination of each waveband signal and the waveband signal is passed to the output port corresponding to the destination. The optical signals can be transmitted to the switch in wavelengths that are unique to the signal destination to avoid the use of wavelength converters in the optical system.
Bragg gratings, either reflective or transmissive, can be included in the waveband selector to switch any number of wavebands. The Bragg gratings of the present invention include one grating produced to reflect an entire waveband or a series of gratings operated in concert that piecewise correspond to the waveband. In an embodiment, tunable permanent Bragg gratings can be provided corresponding to each of the wavebands to allow for dynamic reconfiguration of the switch.
In addition, the optical processing node can include transient gratings to provide for additional reconfiguration of the processing node. Transient grating can be formed in the waveguide either by induction using a coupled circuit or via a writing circuit integrated with the transmission fiber.
In an embodiment of the optical transmission system, pluralities of nodes are interconnected to form a network. The nodes may contain optical transmitters, receivers, add and/or drop devices/ports, and/or switching equipment depending upon whether the node is an origination and/or a destination node, and whether it is a terminal or an intermediate node. In an embodiment, the network management system is provisioned to assign wavelengths to information that can be transmitted to destination nodes in a manner to obviate the need for wavelength conversion at the optical switch. Wavelength assignment can be static or dynamically performed via a network management system, for example, at the client system interface with the optical network. The optical switches cross connecting the nodes and add and/or drop ports are configured to respectively switch and add/drop the information carrying wavelengths in wavebands without separately switching the individual wavelengths.
Accordingly, the present invention addresses the aforementioned problems and provides apparatuses and methods to increase the efficiency and capacity of optical communication systems. These advantages and others will become apparent from the following detailed description.